The described technology generally relates to mapping of logical identifiers to physical identifiers.
Application programs such as a Web browser access resources using the HTTP protocol. The HTTP protocol uses a Uniform Resource Identifier (“URI”) to identify a resource to be accessed. A URI is either a Uniform Resource Locator (“URL”) or Uniform Resource Name (“URN”), or any other formatted string that identifies a network resource. URLs, as an example of URIs, are discussed in detail in T. Berners-Lee et al., Uniform Resource Locators (URL), RVC 1738, CERN, Xerox PARC, Univ. of Minn., December 1994. Each URL contains a domain name identifying where the resource is located. For example, the URL “http://www.odi.com/home.htm” indicates that the resource named “home.htm” is located at the domain of “odi.com.” The HTTP protocol uses an underlying transport layer such as TCP/IP to route messages to the computer associated with the domain name identified in the URL. The TCP/IP protocol, however, identifies computers using IP addresses (e.g., “128.32.0.6”) rather than domain names. Various computer techniques have been developed to map high-level identifiers, such as domain names, to low-level identifiers, such as IP addresses.
One such technique is implemented by the domain name service (“DNS”) that is provided as part of the TCP/IP protocol suite used by the Internet. DNS is a name service that maps domain names to IP addresses. When an application uses the HTTP protocol to send a message to a domain name, it submits a resolution request that specifies the domain name to a local domain name server (e.g., provided by an ISP) that is part of the DNS. The local domain name server may be identified in a configuration file of the client computer executing the browser. The local domain name server checks a local cache to determine whether it has a mapping of that domain name to IP address. If so, it returns the IP address to the application. If not, it forwards the resolution request to a root domain name server. The root domain name server contains a mapping of each top-level domain name (e.g., “com”) to its name server(s). The root domain name server responds to the resolution request by providing the name of the top-level domain name server to the local domain name server. The local domain name server caches the name of the top-level domain name server. The local domain name server then sends the resolution request to the top-level domain name server and receives (and may cache) the name of the domain name server for the second-level domain name (e.g., “odi.com”). The local domain name server forwards the resolution request to the domain name server for the second-level domain name. The domain name server returns an IP address associated with the second-level domain name. Alternatively, it may return the name of the domain name server for the third-level domain name (e.g., “www.odi.com”), and the local domain name server may repeat the process for each level of domain name. Eventually, the local domain name server sends an IP address of the domain name to the computer executing the application. e.g.